Integrated injection logic (I.sup.2 L), also called "merged transistor logic" (MTL), is fast becoming a major digital circuit technology due to its high packing density and very low power dissipation. In implementing an integrated circuit using I.sup.2 L technology it is desirable to combine a relatively large number of I.sup.2 L circuits with significantly fewer vertical coupling transistors which are isolated from the I.sup.2 L circuits. For reasons which will become clear hereinafter, to attain the desired isolation it has been thought necessary to also isolate the emitter region of the I.sup.2 L device from the substrate. This in turn has required that contact be made to the emitter region from the surface of the substrate. Normally, where isolation is not required, contact to the I.sup.2 L emitter would be made through the substrate itself. Due to the relatively large number of I.sup.2 L devices, the additional surface contacts significantly increased the area required per device at the expense of circuit density. Moreover, since contact to the emitter is made near one of its ends, the series resistance in the I.sup.2 L emitter circuit is considerably greater than in the nonisolated I.sup.2 L circuit, in which contact between the substrate and the emitter is over the entire substrate-emitter interface.
It is therefore the principal object of the present invention to devise an improved way to isolate the I.sup.2 L circuits of an integrated circuit from their associated coupling transistors so as to increase circuit density and reduce their series emitter resistance.
The above and other objects of the present invention are attained by providing a P-type isolating shell region between the substrate and each of the coupling transistors of the integrated circuit. While this still requires that a surface contact be made to the emitter of each coupling transistor, it eliminates the need to isolate the I.sup.2 L circuits from the substrate. Hence the need for a surface contact to their emitters is eliminated, since that contact may now be made through the substrate. The elimination of the surface contact from the emitters of the I.sup.2 L devices results in a significant saving of space because there are many more I.sup.2 L devices in the integrated circuits than there are coupling transistors.